The present invention relates to controllers or switch assemblies used for fractional duty cycle modulation of an electrical load, for example an electrical resistance heater employed in an oven or range top "burner" heater. Controllers of this type are employed in domestic cooking appliances; and, typically the duty cycle of the heater is variable by user rotation of a control knob associated with the switch assembly controlling the load current to the heater.
Heretofore, controllers for domestic cooking ovens of the aforesaid type, have utilized a cam which moves with the user rotatable knob for varying the bias on the load current switch which is actuated by a heat motor energized only when load current is flowing through the heater switch. The user rotation of the control knob positions the cam to provide a predetermined bias on the load current switch actuating mechanism which in turn varies the amount of time that the load heater is on. The heat motor includes a resistance which is typically connected for heating a bi-metal actuator which warps in response to heat transfer from the resistance to cause de-actuation of the switch which cuts off of the load current to the oven heater and to the heat motor. In such an arrangement, the time required for the bi-metal actuator to cool upon opening of the load current switch determines the ratio of the "on" time to "off" time.
Known constructions for variable duty cycle load or heater controllers have employed a bi-metal actuator for the load current switch which has a heat motor attached thereto comprising a relatively thin film resistive heater superposed on a rigid substrate and mounted on the bi-metal member. This arrangement of the heat motor has been found generally operative; however, as the bi-metal member is heated by heat transfer from the heat motor, the warpage or deflection induced in the bi-metal member causes the bi-metal to pull away from the rigid heat motor substrate, thereby diminishing the heat transfer from the heat motor to the bi-metal and introducing inaccuracies non-proportionality and hysteresis in the movement of the bi-metal with respect to the energy input to the heater. This warpage of the bi-metal away from the heater has resulted in difficulties in calibrating the heat motor with respect to the actuating point of the load current switch for the energy input to the heat motor. Additionally, the known arrangements for such controllers have applied to the bias from the user rotated cam to one portion of the snap acting load current switch and have applied the bias from the heat motor bi-metal to another portion of the switch. This arrangement has resulted in difficulty in calibrating the controller to provide the desired duty cycle or ratio of "on" time to "off" time of the load heater being controlled and has diminished the repeatability of the switch when calibrated.
Furthermore, known arrangements for the user actuated cam to vary the bias on the load current switch have resulted in numerous parts, complexity and difficulty in calibrating the controller as to the position of the cam with respect to the desired fractional duty cycle of the load heater.
Referring to FIGS. 7 and 10, a known heater controller for domestic cooking ovens is shown wherein the user cam 1 causes a cam follower 2 having one end attached to a blade member 3 cantilevered from stationary structure which causes the cam follower to bias the blade spring 3 in an upward direction. A snap acting blade spring 4 is attached to the blade member 3; and, the spring 4 has a moveable contact 5 on one end thereof which is effective upon movement of the spring 4 for opening and closing against the stationary contact 6. The end of the blade spring 4 is biased downwardly at the end distal contact 5 by the end of a bi-metal member 7, which has attached thereto a heat motor indicated generally at 8, which comprises a relatively thin film resistor mounted on a ceramic substrate 9 with one end of the resistor connected electrically to a cantilevered contact strip 10 with the other distal end thereof contacted by a wiper 11 which is connected to one side of a power line through a cam actuated switch 12. The member 10 and the blade spring 3 are connected to a common member 13 which is connected to the opposite side of the power line from switch 12. A calibration screw 14 which is accessible through a hole in the housing 15 is provided for adjustment of the upright position of the bi-metal actuator 7.
FIG. 7 shows the electrical schematic for the device of FIG. 10 in which the heat motor 8 has one side connected through junction 16 to one side of the switch 12; and, the other side of the heat motor is connected to junction 17 which is connected through a cam actuated switch 18 to a junction 19 which is connected to the opposite side of the power line.
Junctions 16 and 18 are connected through load terminals H1, H2 to the heater load indicated at 22. Switch 12 includes the stationary contact 6 and the movable contact 5 as shown in FIG. 10. The prior art device of FIG. 7 includes a pilot lamp 24 connected through a terminal denoted P and through cam operated switch 20 to junction 19.
Referring to FIG. 8, another known heat motor actuated controller is shown schematically wherein a first and second electrical load heaters 25, 26 are connected in parallel with the second load heater 26 being series connected to the switch 18'.
FIG. 9 shows another known heat motor actuated directional duty cycle heater controller having a single load heater 27 series connected with the heat motor 8 "to the cam actuated switch 12". Heat motor 8 " acts on cam actuated switch 12"in the same manner as in the device of FIG. 7.
The known devices are complex in that many parts are required; in particular, the assembly of the members 8, 9, 7, 10, 13 and 3 in the FIG. 10 prior art device are noted as requiring riveting or weldment and are consequently difficult to assemble in the housing. In addition, the three piece structure of members 2, 3 and 4 is difficult to form as a sub-assembly and install in the housing.
Therefore, it has long been desired to find a way or means of providing a heat motor actuated duty cycle modulating controller for an electrical load such as an electrical resistance heater and to provide such a device which is simple to assemble, has a minimum of parts and is low in manufacturing costs and easy to calibrate is accurately repeatable when calibrated and reliable in operation over extended service life.